A lithium ion secondary battery has been developed for various uses and there has been a demand for performance suitable for various uses ranging from use in a small-sized mobile device to use in a large-sized battery-powered electric vehicle (BEV) and a hybrid electric vehicle (HEV).
For use in a mobile device, with the progress of small-size and lightweight electronic devices as well as the increase in the power consumption due to the diversification of functions, a lithium ion secondary battery having a higher energy density is required.
Further, there is an increasing demand for a secondary battery with a high output and a large capacity for electric tools such as an electric drill and a hybrid automobile. In this field, conventionally, a lead secondary battery, a nickel-cadmium secondary battery, and a nickel-hydrogen secondary battery are mainly used. However, a small and light lithium ion secondary battery with high energy density is highly expected, and there is a demand for a lithium ion secondary battery having excellent large current load characteristics.
In particular, in applications for automobiles, such as battery electric vehicles (BEV) and plug-in hybrid electric vehicles (PHEV), a long-term cycle characteristic over 10 years and a large current load characteristic for driving a high-power motor are mainly required, and a high volume energy density is also required for extending a mileage. Since a large-sized lithium ion secondary battery is expensive, reduction in cost is also required.
Generally, a carbon material such as graphite, hard carbon and soft carbon is used as a negative electrode active material for a lithium ion secondary battery. While hard carbon and soft carbon described in Japanese Patent No. 3653105 (U.S. Pat. No. 5,587,255; Patent Document 1) are excellent in a characteristic with respect to a large current and also have a relatively satisfactory cycle characteristic, the most widely used material is graphite.
Graphite is classified into natural graphite and artificial graphite.
Among those, natural graphite is available at a low cost and has high discharge capacity and electrode filling property due to high degree of graphitization. However, natural graphite has such problems that its particle shape is flake, that it has a high specific surface area, and that it has a significantly low coulomb efficiency at the initial charging and discharging because the electrolyte is decomposed due to highly reactive edge surfaces of graphite, which leading to gas generation. In addition, the cycle characteristics of a battery using natural graphite are not very good. In order to solve those problems, Japanese Patent publication No. 3534391 (U.S. Pat. No. 6,632,569, Patent Document 2) and the like propose a method involving coating carbon on the surface of the natural graphite processed into a spherical shape.
Regarding artificial graphite, there is exemplified graphitized mesocarbon microbeads described in Japanese Patent No. 3126030 (Patent Document 3) and the like.
Graphitized materials made from petroleum pitch, coal pitch, coke and the like is available at a relatively low cost. However, a needle-shaped coke with high crystallinity is flaky and tends to align. In order to solve this problem, the method described in Japanese patent publication No. 3361510 (Patent Document 4) and the like yield results.
In JP 2003-77534 A (Patent Document 5), examination is performed to achieve excellent high-rate charge/discharge characteristics by using artificial graphite having relatively large interparticle spaces.
WO 2011/049199 (U.S. Pat. No. 8,372,373; Patent Document 6) discloses artificial graphite being excellent in cycle characteristics.
Japanese Patent No. 4945029 (U.S. Pat. No. 7,141,229; Patent Document 7) discloses an artificial graphite negative electrode produced from green needle coke having a flow structure which is produced with the addition of boron.
WO 2014/003135 (Patent Document 8) discloses a flaky carbon material in which the surface of a carbon material having specific optical structures is coated.
WO 2014/058040 (US 2015/0263348 A1; Patent Document 9) discloses a carbon material having specific optical structures and containing boron.